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I have never found any of the endings (original series, EoE, or Rebuild) depressing. The entire thesis of Evangelion, the original series, EoE, and all of Rebuild, has always been about stepping out of the warm, umbilical embrace of escapist fantasy into the bright, harsh light of the real world, and Rebuild hammered that point home so hard that Anno used live-action footage as a visual metaphor in that final scene. It was never about Shinji's self-actualization, it was about the audience's. Shinji's ultimate happiness was only intended to convey that there are things which make the real world, despite its faults and difficulties, worth inhabiting. They didn't need to create a world better than our own, that would be yet another retreat into fantasy. We don't live in global communism, and we can't build it if we spend all day dreaming instead of acting.
I also think it was the kindest presentation all of the characters have been given in any form. We got a chance to understand and empathize with Gendo, of all people.